The invention relates generally to catheters, and more particularly, to locking mechanisms for steerable catheters.
The heart beat in a healthy human is controlled by the sinoatrial node ("S-A node") located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of or damage to the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as cardiac arrhythmia.
Electrophysiological ablation is a procedure often successful in terminating cardiac arrhythmia. This procedure involves applying sufficient energy to the interfering tissue to ablate that tissue thus removing the irregular signal pathway. However, before an ablation procedure can be carried out, the interfering tissue must first be located.
One location technique involves an electrophysiological mapping procedure whereby the electrical signals emanating from the conductive endocardial tissues are systematically monitored and a map is created of those signals. By analyzing that map, the interfering electrical pathway can be identified. A conventional method for mapping the electrical signals from conductive heart tissue is to percutaneously introduce an electrophysiology ("EP") catheter having mapping electrodes mounted on its distal extremity. The catheter is maneuvered to place those electrodes in contact with or in close proximity to the endocardium of the patient's heart. By monitoring these signals, aberrant conductive tissue sites responsible for the arrhythmia can be pinpointed.
Once the origination point for the arrhythmia is located, the physician may use an ablation procedure to restore normal heart beat or at least improve the heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels.
In order to perform the above described electrophysiological procedures, the EP catheter, having mapping and ablation electrodes at the distal end thereof, is percutaneously introduced into the cardiovascular system of the patient through a blood vessel, for instance the femoral artery, and advanced to a selected endocardial site in the atrium or ventricle. Once the EP catheter has reached the heart, it must be maneuvered to place its electrodes or other active devices in the required positions to perform the EP procedures.
A steerable catheter may be used that has an internal deflection control line attached at a point adjacent the distal tip of the catheter. Pulling the proximal end of the control line at the manipulation handle causes the distal tip of the catheter to bend in one direction so that the tip may be directed to a selected endocardial site. The ability to position these devices at the target site is important to obtaining good ablation results.
Most steerable catheters have catheter tubes or shafts formed of an elongated flexible, yet resilient material, so that when the tube is in its relaxed state, it tends to maintain a generally straight configuration. The tension on the control line overcomes the natural resilient straightening forces so that a deflection and a curved configuration results. The natural resilient straightening forces of the catheter tube oppose deflection forces applied by the physician to the control wire. If those straightening forces are strong enough, the physician may need to continuously apply pressure to the control wire to maintain a deflection of the distal end. Requiring the physician to apply continuous and substantial force to hold a distal end deflection can cause tiring as well as decreased accuracy in maintaining a certain deflection.
In order to free the physician's hand from having to continuously apply force, it is therefore desirable for the steerable EP catheter to include a locking mechanism that holds the tip deflection control line at the position selected to maintain the catheter distal end curvature and oppose the resilient straightening force of the catheter tube. Such a feature would be useful for maintaining intimate electrode contact with a particular selected endocardial tissue site during mapping and ablation procedures. The curvature and position of the distal portion of the catheter may have to be finely adjusted one or more times in order to facilitate complete and comprehensive monitoring of the electrical signals emanating from the conductive heart tissue for effective mapping and detection of arrhythmatic points of origin. In addition, in order to foster precise location of the ablation electrode adjacent selected endocardial treatment sites, frequent but minute repositioning of the tip electrode thereof may often be required to facilitate accurate effective ablation of the aberrant heart tissue. Therefore, a deflection control line locking mechanism should provide increased repositioning and locking resolution, allowing for fine catheter distal end curvature control.
One control line locking mechanism utilizes a toothed lever pivoted on a sleeve, the lever selectively engaging ratchet threads disposed on a steerable catheter handle. The catheter handle is connected to the proximal end of a deflection control line so that pulling the handle longitudinally relative to the sleeve deflects the distal end of the catheter and the toothed lever on the sleeve engages the ratcheted threads to hold the control line in position. Although this type of mechanism has been found effective in certain instances, locking resolution and therefore distal end curvature control is restricted to the resolution defined by the ratchet spacing of the handle, which can be rather coarse.
Another deflection control line locking mechanism uses an externally threaded control member attached to the proximal end of the control line. The externally threaded member is threadedly engaged to an internally threaded collar, whereby rotation of the collar relative to the member, pulls the control line and threaded friction therebetween holds the control line at its selected position. Although this locking device allows for increased catheter distal end curvature resolution, many revolutions of rotation may be required to affect the curvature, which in turn can slow down the catheter steering process.
Yet in another control line locking mechanism, O-rings are disposed between the proximal end of the catheter tube and the manipulation handle. The manipulation handle is attached to the proximal end of the control line so that pulling the handle longitudinally relative to the catheter tube deflects the distal end thereof. The O-ring holds the handle relative to the catheter tube so that the control line device is held in the selected position. However, O-rings may "creep" over time and use and not retain the frictional contact between surfaces required to effectively lock those surfaces relative to one another. Furthermore, should bodily fluids, or the like, come into contact with the surface of the O-ring, the O-ring may tend to slip releasing the control line from its locked position. In addition, the O-ring may tend to twist and rip as the control line device is moved relative to the manipulation handle. Moreover, if the O-ring is constructed of an overly elastic material the O-ring may tend to adhere to the manipulation handle and bind or stick the handle and catheter relative to one another.
In one steerable catheter, a stiffening member or mandrel is also located in the catheter tube in addition to the control line, and includes a stiffening member control device mounted to the manipulation handle that may be moved relative to the catheter tube and control line to stiffen the catheter shaft and to alter the radius of bend at the distal end. For instance, a mandrel may be moved towards the distal end or more towards the proximal end of the catheter to alter the radius of bend at the distal end, providing an additional degree of selective steering control.
Where a threaded element is used to advance and retract the mandrel, the frictional properties of the threaded element sufficiently lock the mandrel in its selected position overcoming the natural straightening forces of the catheter tube. It is desirable that the deflection control line locking mechanism not interfere with the independent rotatable operating and locking characteristics of the threaded element controlling the stiffening member.
Hence, those skilled in the art have recognized the need for a steerable catheter having the ability to lock the distal end deflection of the catheter at the selected position. In addition, the deflection control line locking mechanism should provide increased locking resolution allowing for freer catheter distal end curvature control. Furthermore, the deflection control line locking mechanism should not interfere with the operation of other steering control mechanisms and should be relatively easy to operate, inexpensive to manufacture, and reliable in use. The present invention fulfills these needs and others.